Heretofore trucks have been proposed for use on tracks of different gage width. Some have used separable side frames and separate axle parts; some have used separable axle parts slidable in side frames; some have used wheels slidable on axles; some have used separable trucks for four-rail tracks; and some have used shiftable wheel-axle units swingable into position for alternate use. But so far as is known, none has provided for gage shift while maintaining side frames at fixed width apart and axle units of fixed length with the same fixed bearing supports in the frames.